Responding to the suburban exodus of its members, the historic Har Sinai congregation has selected a site in Worthington Valley for its move from Northwest Baltimore to Baltimore County, officials of the temple said yesterday.

Har Sinai, the nation's oldest Reform Jewish congregation, plans to buy a 13 1/2 -acre parcel at Greenspring and Walnut avenues under a contract approved Sunday by its board of trustees.

"It was felt to be in a central, convenient location for our membership, both now and where we project our membership to be in the future," said Dr. Robert K. Brookland, congregation president.

Most members who have joined Har Sinai in recent years live in the Reisterstown or Owings Mills areas or in Cockeysville and other communities farther east, he said.

Dr. Brookland also said the property should provide a serene setting for worship. Chosen after a membership survey, it was among four properties seriously considered.

In January, members voted overwhelmingly to sell Har Sinai's property in Baltimore, prompting congregation leaders to begin searching for sites in the county.

The vote was scheduled after Har Sinai received an offer from an Orthodox Jewish organization, the Maimonides Academy, to buy the temple's property for $4 million.

An administration building and classrooms will be built first. Meanwhile, the congregation, which includes about 600 families, will continue to worship at its sanctuary at 6300 Park Heights Ave. in Baltimore.

Dr. Brookland said construction probably would not begin until 1997, after engineering and design studies are complete.

Geoffrey Glazer, head of the congregation's site committee, said Har Sinai would buy part of a 17.7-acre parcel, and would assist the current owner in subdividing the rest of the land into three lots. No purchase price was disclosed.

Mr. Glazer and Dr. Brookland said the contract approved by the trustees contains terms that already have been negotiated with the seller. Geoffrey L. Forman, a lawyer representing the property owner, said he had not seen the signed contract. "If it's the contract I've discussed with [the congregation's] counsel, it's all been agreed upon."